Method and system for providing automatic gate bias and bias sequencing for field effect transistors

ABSTRACT

A feedback gate bias circuit for use in radio frequency amplifiers to more effectively control operation of LDFET, GaNFET, GaAsFET, and JFET type transistors used in such circuits. A transistor gate bias circuit that senses drain current and automatically adjusts or biases the gate voltage to maintain drain current independently of temperature, time, input drive, frequency, as well as from device to device variations. Additional circuits to provide temperature compensation, RF power monitoring and drain current control, RF output power leveler, high power gain block, and optional digital control of various functions. A gate bias circuit including a bias sequencer and negative voltage deriver for operation of N-channel depletion mode devices.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application claims priority to U.S. patent application Ser.No. 14/149,468, filed Jan. 7, 2014, entitled “METHOD AND SYSTEM FORPROVIDING AUTOMATIC GATE BIAS FOR FIELD EFFECT TRANSISTORS,” whichapplication is to issue Feb. 24, 2015 as U.S. Pat. No. 8,963,643, toU.S. Pat. No. 8,624,675, issued Jan. 7, 2014, entitled “METHOD ANDSYSTEM FOR PROVIDING AUTOMATIC GATE BIAS FOR FIELD EFFECT TRANSISTORS,”to U.S. Pat. No. 8,188,794, issued May 29, 2012, entitled “METHOD ANDSYSTEM FOR PROVIDING AUTOMATIC GATE BIAS FOR FIELD EFFECT TRANSISTORS,”and to U.S. Provisional Patent Application No. 61/340,960, filed Mar.25, 2010, entitled “METHOD AND SYSTEM FOR PROVIDING AUTOMATIC GATE BIASFOR FIELD EFFECT TRANSISTORS,” as all of which are hereby incorporatedby reference herein in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to apparatus and method foramplifying radio frequency (RF) signals, including microwave RF signals.More particularly, the present invention pertains to a transistor gatebias circuit for RF amplifier applications to overcome device variationsand changing operating conditions to control and maintain transistordrain current.

BACKGROUND OF THE INVENTION

The present invention provides an RF amplifier gate bias circuit that isappropriate for use in a wide range of frequencies and applicationshaving no particular bounds and including KHz to GHz, including in theL, S, and C bands. The S band ranges from 2 to 4 GHz and is part of themicrowave band of the electromagnetic spectrum used in weather radar,surface ship radar, and communications satellites applications. The Lband, referred to as the IEEE L band, is a portion of the microwave bandof the electromagnetic spectrum ranging from 1 to 2 GHz. The L band isused in communications, digital audio broadcast, satellitecommunications, telecommunications, military, telemetry as well as otherapplications. For instance, the Global Positioning System (GPS) utilizescarriers in the L band. Uses for IEEE C-band frequencies, which extendfrom 4 to 8 GHz, include satellite communications, weather radar, andmilitary applications.

Laterally Diffused Field Effect Transistor (LDFET), also referred to asLaterally Diffused Metal-Oxide Semiconductor (LDMOS), type RadioFrequency (RF) devices have several advantages over bipolar transistorssuch as higher gain, higher efficiency, and wider dynamic range ofoutput power. LDFETs also have a major disadvantage in that the gatebias voltage (V_(g)) required to set the quiescent current (I_(d))drifts over temperature, time, input drive, and frequency, as well asfrom device to device variations. Considerable effort has been expendedby the various manufacturers of these devices to lessen this undesirableeffect, but no one has fully solved the problem.

Exemplary uses of the RF amplifier gate bias circuit of the presentinvention are transmission applications, including transmitters,receivers, and power amplifiers.

What is needed is a solution to address the various undesirableoperational side effects associated with use of LDFET, GaNFET, GaAsFET,JFET and other such transistors to more fully and efficiently takeadvantage and utilize their beneficial properties and to expand theacceptable use of such devices in a wider range of RF applications.

In addition, N-Channel depletion device-based amplifiers operate withthe negative characteristic of N-channel depletion mode devices thatrequre a negative gate voltage and gate-drain bias sequencing for properoperation. With any N-Channel depletion device, such as GaAs FET, GaNFET, or N-channel silicon junction FET, it is essential that thenegative gate voltage arrives before the drain voltage otherwise thedrain to source resistance is a very low value which will essentiallyshort out the input power and likely cause damage to several circuitcomponents including the depletion device. Existing approaches tosequencing for GaN devices, e.g., test fixture set up for fire-up andshut-down sequencing, are cumbersome and are external to the device,e.g., amplifier. For example, supplying a negative voltage on a testfixture or lab bench is typically accomplished with an external supplyhaving negative voltage generation capability or by switching the leadsbetween the ground node and the positive voltage node. In an applicationcircuit the negative voltage comes from a regulator or a negativevoltage generator. The goal in bias sequencing the device is to avoidareas that are sensitive to potential instability of the device, e.g.,the area where V_(DS) drain to source is low and I_(DS) drain to sourceis high. What is needed is an improved sequencer for use in RFamplifiers employing N-channel depletion mode devices that is internalto the amplifier circuit or device and that is flexible in accommodatinga variety of such devices having differing attributes.

Applications for the invention include two-way private radiocommunication, broadband amplifiers, cellular infrastructure, testinstrumentation, and Class A, AB, Linear amplifiers suitable for OFDM,W-CDMA, EDGE, CDMA waveforms.

As discussed above, temperature compensation is another aspect tocircuit integrity and this has further relevance to bias sequencing andto adequately maintain the bias of the device for consistent performanceover temperature. The quiescent current of a GaN HEMT device isprimarily a function of temperature and V_(GS). What is needed is a biascircuit with temperature compensation that can maintain consistentoperational performance over a prescribed range of temperaturefluctuation, e.g., −50 to 100 degrees Celsius.

SUMMARY OF THE INVENTION

The present invention is intended for many uses and applicationsincluding in design and manufacture of airborne and ground-basedtelemetry equipment, including aircraft (manned and unmanned), groundvehicles, fixed systems and military telemetry equipment. Telemetrysystem components include transmitters, receivers, and power amplifiersin a wide variety of frequency ranges. As in many areas, there is agrowing need and desire for telemetry components that are low cost, lowpower consumption (for battery, heat and other concerns), light weight,low failure rate, less complex, compact, more robust and rugged designfor harsh environments, and of course high performance. For instance,the RF amplifier gate bias circuit of the present invention may beincorporated in transmitters, receivers, and power amplifiers.

In one aspect, the invention provides a transistor gate bias circuit forRF amplifiers that senses drain current and automatically adjusts orbiases the gate voltage to maintain drain current independently oftemperature, time, input drive, frequency, as well as from device todevice variations.

In another aspect of the invention a major advantage over prior art isthat unlike conventional gain blocks, the supply current variesaccording to the output power required to maintain a constant gain.

In yet another aspect of the invention an advantage over prior circuitsis that it does not attempt to minimize the spurious responses by betterdecoupling or improved grounding or any of the other known techniques.Rather, the circuit of the invention eliminates the problem entirely byshutting down the negative voltage deriving oscillator once it is nolonger needed.

In a further aspect, the present invention provides an improvedN-Channel depletion device-based amplifier with novel gate bias circuitand sequencer. With any N-Channel depletion device, such as GaAs FET,GaN FET, or N-channel silicon junction FET, it is essential that thenegative gate voltage arrives before the drain voltage or the drain tosource resistance is a very low value which will essentially short outthe input power and likely cause damage to several circuit componentsincluding the depletion device. The present invention provides a novelsequencer for more effectively maintaining the necessary condition toprevent damage to amplifier components. The present invention may alsoprovide an Adaptive Drain Current Control (ADCC). In a further aspect,an opto-coupler may be used in the circuit or in the alternative abattery or a Peltier Effect thermoelectric device.

In one embodiment, the present invention involves an RF amplifiercircuit comprising: a FET for receiving a RF input signal and generatingan amplified RF output signal, the FET having a gate, drain, and source;a control circuit, connected to the gate and drain of the FET, forcontrolling the current at the drain; and a bias circuit comprising ameans for biasing and variably compensating drift in the gate thresholdvoltage required to set the quiescent drain current, the bias circuitbeing connected to the control circuit and controlling operation of thecontrol circuit to maintain constant current at the drain at wake-uptransition; whereby the output remains essentially constant relative toexternal temperature.

Further, the present invention may comprise temperature-sensing means,connected to the control circuit, for sensing change in temperature; andthermal compensation means, connected to the temperature-sensing meansand control circuit, for automatically adjusting the drain current ifthe temperature decreases or increases to maintain essentially constantoutput with respect to temperature; whereby the control circuitmaintains essentially constant current at the drain with respect totime, input drive, frequency, and device-to-device variations, but nottemperature. The present invention may be adapted to provide essentiallyconstant output power throughout operation, including wake-up transitionand post-wake-up transition operation. The thermistor of the presentinvention may comprise both the temperature-sensing means and thethermal compensation means.

The present invention may also comprise: detecting means, connected tothe RF input signal, for detecting the power level of the RF inputsignal and supplying a DC voltage representative of the detected powerlevel; means for producing a variable reference voltage; comparingmeans, having an input for receiving the variable reference voltage andbeing connected to the detecting means, for comparing the supplied DCvoltage to the variable reference voltage; and switching means,connected to the comparing means and the bias circuit, for turning offthe LDFET if the input signal level is less than the variable referencevoltage; wherein disposed intermediate of the RF input signal and thedetecting means is one of a group consisting of a capacitor and acoupler.

Moreover, the present invention may further comprise: detecting means,connected to the RF output signal of the circuit, for detecting thelevel of the output signal and supplying a DC voltage representative ofthe detected output signal level; means for producing a variablereference voltage; and adjusting means, connected to the detectingmeans, the variable reference voltage and the bias circuit, forautomatically increasing or decreasing the drain current if the suppliedDC voltage is lower or higher (respectively) than the reference voltageto maintain essentially constant output RF power; wherein the disposedintermediate of the RF output signal and the detecting means is one of agroup consisting of a capacitor and a coupler.

Additionally, the present invention may further comprise: a digitalreference voltage generator adapted to produce and output a digitalsignal representing the reference voltage; and a digital-to-analogconvertor having an input for receiving the digital signal, and anoutput connected to the adjusting means for supplying acomputer-controllable analog reference voltage signal to the adjustingmeans.

The present invention may also further comprise: a first detectingmeans, connected to the RF input signal of the circuit, for detectingthe power level of the input signal and supplying a DC voltagerepresentative of the detected input signal level; and a seconddetecting means, connected to the RF output signal of the circuit, fordetecting the level of the output signal and supplying a DC voltagerepresentative of the detected output signal level; and an adjustingmeans, connected to the first detecting means, the second detectingmeans and the bias circuit, for automatically increasing or decreasingthe drain current if the second supplied DC voltage is lower or higher(respectively) than the first supplied DC voltage by an amount necessaryto maintain an essentially constant gain; wherein disposed intermediateof the RF input signal and the detecting means is one of a groupconsisting of a first capacitor and a first coupler, and whereindisposed intermediate of the RF output signal and the detecting means isone of a group consisting of a second capacitor and a second coupler.

Also, the present invention may further comprise: a digital attenuator,connected to the RF input signal and the first detecting means, foradjusting a gain of the circuit. In addition, the present invention mayalso involve comprising: means for producing a negative voltage signal;a voltage regulator having an input and an output, the input connectedto the negative voltage producing means and the drain of the FET, theoutput connected to the gate of the FET, the voltage regulator adaptedto supply a regulated negative voltage signal to the gate of the FET;and a shutdown means, connected to the negative voltage producing means,for shutting down the negative voltage producing means after a FETwake-up transition; and whereby the voltage regulator supplies aregulated negative voltage signal to the gate of the FET both during andafter the FET wake-up transition; wherein the voltage regulatorcomprises: an inverting amplifier comprising an operational amplifier,the negative supply of the operational amplifier being connected to thenegative voltage producing means and the drain of the FET; wherein themeans for biasing and variably compensating drift comprises a variableresistance device; and wherein the FET is one of a group consisting ofLDFET, GaNFET, GaAsFET, JFET, and MOSFET.

In yet another embodiment, the present invention provides an RFamplifier circuit comprising: a FET for receiving a RF input signal andgenerating an amplified RF output signal, the FET having a gate, drain,and source; a control circuit, connected to the gate and drain of theFET, for controlling the current at the drain; a dividing circuitcomprising a means for biasing and variably compensating drift in thegate threshold voltage required to set the quiescent drain current, thedividing circuit being connected to the control circuit and controllingoperation of the control circuit to maintain an essentially constantcurrent at the drain in connection with a wake-up transition; adetecting means, operably connected to the RF output signal, fordetecting the power level of the RF output signal and supplying a DCvoltage representative of the detected output power level; a means forproducing a variable reference voltage; and an adjusting means,connected to the detecting means, the variable reference voltage and thedividing circuit, for automatically adjusting the drain current based atleast in part on a comparison of the supplied DC voltage and thereference voltage by an amount necessary to maintain essentiallyconstant output RF power.

Also the present invention may further comprise: a small valuecapacitor, operably connected to the RF output signal and the detectingmeans. In addition, the present invention may also involve comprising:wherein the reference voltage is a digital computer-controlled referencevoltage input, and further comprising a digital-to-analog convertoroperably connected to the digital input and the adjusting means andadapted to supply a computer-controllable analog reference voltagesignal to the adjusting means.

In yet another embodiment, the present invention provides a methodcomprising: receiving by a FET a RF input signal and generating anamplified RF output signal, the FET having a gate, drain, and source;controlling the current at the drain by biasing and variablycompensating drift in the gate threshold voltage required to set thequiescent drain current to maintain an essentially constant current atthe drain in connection with a wake-up transition; and based at least inpart on temperature change, automatically altering the drain current tomaintain essentially constant output power with respect to temperature;maintaining essentially constant drain current with respect to time,input drive, frequency, and device-to-device variations, while allowinga change in drain current with respect to temperature variations.

In yet another embodiment, the present invention provides an RFamplifier circuit comprising: a FET for receiving a RF input signal andgenerating an amplified RF output signal, the FET having a gate, drain,and source; a control circuit, connected to the gate and drain of theFET, for controlling the current at the drain; a bias circuit comprisinga means for biasing and variably compensating drift in the gatethreshold voltage required to set the quiescent drain current, the biascircuit being connected to the control circuit and controlling operationof the control circuit to maintain an essentially constant current atthe drain; a deriving means for deriving a negative voltage signal; anda regulating means having an input and an output, and operably connectedat the input to the deriving means and operably connected at the outputto the gate of the FET, and supplying a regulated negative voltagesignal to the gate of the FET, whereby the regulating means supplies aregulated negative voltage signal to the gate of the FET. Additionalfeatures of the invention may include: a shutdown means, connected tothe deriving means, for shutting down the deriving means after astart-up mode of the amplifier; or the regulating means may comprise aninverting amplifier comprising an operational amplifier, the negativesupply of the operational amplifier being connected to the derivingmeans and the drain of the FET; or the deriving means comprises eitheran optically coupled negative generator or an oscillator negativegenerator; or the FET is an N-Channel depletion mode device; or a biassequencer adapted to maintain the FET in pinch-off condition before thedrain voltage is applied to avoid the FET acting as a short circuit; orthe bias sequencer comprises a low drop out voltage regulator; or thebias sequencer comprises a P-channel MOSFET; or an adaptive currentcontrol circuit adapted to measure input RF power and to output a signalrepresenting the input RF power, whereby during operation of the RFamplifier circuit an increase in input RF power causes the drain currentof the FET to increase and a decrease in input RF power causes the draincurrent of the FET to decrease; or an adaptive current control circuitadapted to receive a signal representing output RF power, whereby duringoperation of the RF amplifier circuit an increase in output RF powercauses the drain current of the FET to increase and a decrease in outputRF power causes the drain current of the FET to decrease; or a means forswitching on and off the drain current of the FET, whereby drain currentis permitted only when input RF power is sensed. Further, the circuitmay comprise: detecting means, operably connected to the RF inputsignal, for detecting the power level of the RF input signal andsupplying a DC voltage representative of the detected power level; meansfor producing a variable reference voltage; comparing means, operablyconnected to the detecting means and the variable reference voltage, forcomparing the supplied DC voltage to the variable reference voltage; andmeans, operably connected to the comparing means, for furthercontrolling operation of the FET based at least in part on a comparisonof the supplied DC voltage and the variable reference voltage. Thecircuit may further comprise: a first detecting means, operablyconnected to the RF input signal, for detecting the power level of theRF input signal and supplying a first DC voltage representative of thedetected input power level; a second detecting means, operably connectedto the RF output signal of the circuit, for detecting the power level ofthe RF output signal and supplying a second DC voltage representative ofthe detected output power level; and an adjusting means, connected tothe first detecting means, the second detecting means and the biascircuit, for automatically adjusting the drain current based at least inpart on a comparison of the second supplied DC voltage and the firstsupplied DC voltage by an amount necessary to maintain an essentiallyconstant gain. The circuit may further comprise: detecting means,operably connected to the RF output signal of the circuit, for detectingthe power level of the RF output signal and supplying a second DCvoltage representative of the detected output power level; and means forproducing a variable reference voltage; adjusting means, connected tothe detecting means, the variable reference voltage means and the biascircuit, for automatically adjusting the drain current based at least inpart on a comparison of the supplied DC voltage and the referencevoltage to maintain an essentially constant output RF power.

A further embodiment of the invention provides an RF amplifier circuitcomprising: an N-Channel depletion mode FET for receiving a RF inputsignal and generating an amplified RF output signal, the FET having agate, drain, and source; a control circuit, connected to the gate anddrain of the FET, for controlling the current at the drain; a biascircuit comprising a means for biasing and variably compensating driftin the gate threshold voltage required to set the quiescent draincurrent, the bias circuit being connected to the control circuit andcontrolling operation of the control circuit to maintain an essentiallyconstant current at the drain; a means for supplying a regulatednegative voltage signal to the gate of the FET; and a bias sequenceradapted to maintain the FET in pinch-off condition before a drainvoltage is applied to avoid the FET acting as a short circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a full understanding of the present invention,reference is now made to the accompanying drawings, in which likeelements are referenced with like numerals. These drawings should not beconstrued as limiting the present invention, but are intended to beexemplary and for reference.

FIG. 1 is a schematic of a prior art LDFET-based Radio Frequency (RF)amplifier;

FIG. 2 is a schematic of a first embodiment of a LDFET amplifier of thepresent invention with the automatic gate bias circuit;

FIG. 3 is a schematic of an optional temperature compensation circuitfor use with the amplifier of FIG. 2;

FIG. 4 is a variation of the amplifier of FIG. 2 having an RF powermonitoring and drain current control circuit;

FIG. 4A is an alternative RF Input coupler arrangement for use in thecircuit of FIG. 4;

FIG. 5 is a variation of the amplifier of FIG. 2 having an RF outputpower leveling circuit;

FIG. 6 is an optional digital input circuit for use with the RF outputpower leveling circuit in the amplifier of FIG. 5;

FIG. 7 is a variation of the amplifier of FIG. 2 having high power gainblock configuration;

FIG. 8 is an optional digital attenuator for digital control for usewith the high gain block variation of FIG. 7;

FIG. 9 is a schematic of a prior art GaNFET-based amplifier;

FIG. 10A is a schematic of a first half of a first embodiment of aGaNFET-based amplifier of the present invention with gate bias circuitfor connection with the remaining circuitry of FIG. 10B;

FIG. 10B is a schematic of a second half of a first embodiment of aGaNFET-based amplifier of the present invention with gate bias circuitfor connection with the remaining circuitry of FIG. 10A;

FIG. 11 is a schematic of an RF power monitoring and drain currentcontrol circuit for use with the amplifier of FIGS. 10A/10B;

FIG. 12 is a schematic of an RF output power leveler circuit for usewith the amplifier of FIGS. 10A/10B;

FIG. 13A is a schematic of an alternative second embodiment of aGaNFET-based amplifier of the present invention with gate bias circuitfor connection with the remaining circuitry of FIG. 13B;

FIG. 13B is a schematic of a second half of the alternative secondembodiment of a GaNFET-based amplifier of the present invention withgate bias circuit for connection with the remaining circuitry of FIG.13A;

FIG. 13C is an alternative transistor base input circuit for the circuitof FIG. 13B;

FIG. 14 is a schematic of a first embodiment of an improved N-Channeldepletion device-based amplifier of the present invention with gate biascircuit and sequencer;

FIG. 15 is a schematic of a second embodiment of an improved N-Channeldepletion device-based amplifier of the present invention with gate biascircuit and sequencer;

FIG. 16 is a schematic of the circuit of FIG. 15 having a opticallycoupled negative generator circuit;

FIG. 17A is a schematic of a first half of a third embodiment of animproved N-Channel depletion device-based amplifier of the presentinvention with gate bias circuit and sequencer and having an adaptivecurrent control circuit;

FIG. 17B is a schematic of a second half of the circuit of FIG. 17A;

FIG. 18A is a schematic of a first half of an alternative circuit ofFIG. 17 A/B having an output RF power detecting and feedback circuit;

FIG. 18B is a schematic of a second half of the circuit of FIG. 18A;

FIG. 19A is a schematic of an alternative second embodiment of anadaptive current control circuit to that of FIG. 17 A/B;

FIG. 19B is a schematic of a second half of the circuit of FIG. 19A; and

FIGS. 20A-B are illustrations of a series of curves A-F related tooperation of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described in more detail withreference to exemplary embodiments as shown in the accompanyingdrawings. While the present invention is described herein with referenceto the exemplary embodiments, it should be understood that the presentinvention is not limited to such exemplary embodiments. Those possessingordinary skill in the art and having access to the teachings herein willrecognize additional implementations, modifications, and embodiments, aswell as other applications for use of the invention, which are fullycontemplated herein as within the scope of the present invention asdisclosed and claimed herein, and with respect to which the presentinvention could be of significant utility.

The following exemplary circuits contain values which are typical foroperation at +20 VDC and in the 2 to 3 GHz band and help explain theinventive contribution in terms of performance. Although the inventionmay be described in examples in the GHz range, it should be understoodthat the invention has broad application in RF applications including inthe KHz range. LDFETs generally have application up to 3 GHz while GaAsand GaN FETs typically have application up to 20-30 GHz. Althoughparticular parts and values are shown, these are by way of example tohelp explain the invention and are not to be considered limiting to theinvention.

FIG. 1 illustrates a prior art Radio Frequency (RF) Amplifier 100 havinga supply voltage of +20Vdc 102 and which receives an RF signal input 118and outputs an amplified RF signal 132. Input Matching Network 122,which can include capacitors, inductors, resistors, and transmissionlines, is designed to provide required impedance matching, harmonicattenuation, bandwidth and other operating characteristics for use atoperating frequencies of circuit 100 and in particular in matching thepower transistor Q1. This matching circuit may be calculated usingmathematical models and simulation programs. In the circuit of FIG. 1the gate of RF power transistor Q1 124 is biased by adjusting thepotentiometer R2 110 to cause the desired quiescent drain current toflow. In this example, Q1 124 is a 10 Watt LDMOS (Laterally DiffusedMetal-Oxide Semiconductor) power transistor for applications atfrequencies between 2.5 and 2.7 GHz. For example, Q1 124 may be partnumber BLF6G27-10 supplied by NXP Semiconductors. In this example,diodes 104, 106,112, 114 (CR1-4) are part number 1N4148 of NXPSemiconductors. By adding or removing diodes 104, 106, 112, 114 (CR1-4)in series with resistors 108 and 110 (R1 and R2), the gate voltage canbe made to increase or decrease as a function of temperature, therebycompensating the drift of the gate threshold voltage with respect totemperature on an open loop basis. Determining the proper temperaturecompensation for each device requires several temperature cycles of upto four hours each. This is obviously a time-consuming procedure tyingup expensive test equipment as well as an RF technician for many hoursper amplifier. Moreover, in the end this does nothing to solve the otheraspects of the gate drift problem.

The amplifier 200 of FIG. 2 solves the gate drift problem by providing adrain current or gate bias control circuit 201, including drain currentsensing resistor R1 204, that senses the drain current Id 227 of Q2 228and automatically adjusts the gate voltage Vg 229 to maintain draincurrent Id 227 independently of temperature, time, input drive,frequency, as well as from device to device variations. Q2 228, like Q1124 of FIG. 1 above, is in this example a 10 W LDMOS or LDFET powertransistor. With the circuit as shown, adjusting the potentiometer R3208 from end to end will cause the drain current Id 227 to vary from 650mA to 1.60 A and, once set, Id 227 will then remain essentially constantwith respect to the above parameters. In this manner, a user mayincrease of decrease the setting of drain current Id 227 by adjustingthe potentiometer R3. Assume a supply voltage of +20.0 VDC 202 and aforward drop over diode CR1 212 of 0.70 VDC. The cathode 213 of diode212 will then be at a potential of 19.30 VDC. The purpose of diode 212is to temperature compensate the base (B) to emitter (E) voltage (Vbe)of Q1 214. In this example, Q1 214 has part number 2N2907 of NXPSemiconductors, also known as PN2907, and is a bipolar transistorintended for low power amplifying or switching applications. The wiper209 of potentiometer R3 208, which is connected to the base of Q1 214,will then allow adjustment from +18.98 VDC to +19.17 VDC. With the wiper209 of potentiometer 208 centered, the base will be at +19.07 VDC.Assuming a Vbe of Q1 of 0.70 VDC, the emitter of Q1 will then be at+19.77 VDC. For Q1 emitter to be at +19.77 VDC, the drain of Q2 mustdraw 1.15 A (1.15 A×0.2 ohms=0.23 VDC=20 VDC−19.77 VDC). For this tohappen, a current of approximately 1 mA must and will flow from thecollector (C) of Q1 to develop sufficient voltage across resistor R6 226to bring the gate of Q2 228 up to the voltage required to cause 1.15Amps, Id 227, to flow through its drain.

Should Q2 228 attempt to draw more current, the base to emitter voltage(Vbe) of Q1 214 will be lowered, causing less current to flow to R6 226,causing the gate voltage to lower, thereby lowering the drain current Id227 of Q2. The opposite is true should Q2 attempt to draw less draincurrent.

Inductors L1 218 and L2 230 act as RF chokes preventing RF power fromreaching the collector of Q1 214 or the supply voltage 202. CapacitorsC1 222 and C2 232 isolate the gate (G) and drain (D) of Q2 228 from a DCstandpoint from the input and output matching circuitry, 224 and 234respectively. Resistor R5 216 acting in concert with resistor R6 226forms a voltage divider which prevents the collector (C) of Q1 214 fromsupplying excessive gate voltage to Q2 during “wake up.” This initial“wake-up” transition or period may be in the context of the transistoroperation or the overall circuit operation and may occur at initialstart-up and/or at other periods during operation of the circuit or acircuit or system in which the amplifier is used. For instance, theoverall circuit may be turned off or go into a “sleep” or “stand-by”mode of operation in power management to conserve energy and extendlifespan. The “wake-up” aspect of operation of either a transistor or acircuit is known by those skilled in the art of amplifier design anduse.

FIG. 3 represents an optional thermal or temperature compensationcircuit 300 for incorporation into the circuit of FIG. 2 as shown atconnection nodes 301 and 302. The exemplary circuit of FIG. 3 includestwo resistors 304 and 306 (R8, R9) and a thermistor 308 (R7) that areadded to provide the opportunity to intentionally change the draincurrent Id 227 of Q2 228 as a function of temperature. It is oftendesirable to increase the drain current Id 227 as the temperatureincreases in order to maintain a constant output power. With the valuesshown, and the ambient current set to 1.15 Amps, this would increase toapproximately 1.50 Amps at +70 C. However, when the drain current isallowed to change according to temperature, the function of the controlcircuit varies slightly. Whereas in FIG. 2 the control circuit'sfunction is to draw a constant current at the drain, its function inFIG. 3 is to draw a constant current at the drain with respect to allparameters except for temperature. By allowing the drain current Id tovary according to temperature, a constant output can be maintainedwithin a range of different temperatures. This advantage is not achievedin the first embodiment of the invention. In the example of FIG. 2, thegate bias control circuit 201 senses the drain current Id 227 of Q2 228and automatically adjusts the gate voltage Vg 229 to maintain draincurrent Id 227 independently of temperature, time, input drive,frequency, as well as from device to device variations. Adjusting thepotentiometer R3 208 causes the drain current Id 227 to vary from, e.g.,650 mA to 1.60 A and, once set, Id 227 will remain essentially constant.The temperature compensation circuit of FIG. 3 is used to introduce achange to separately adjust and manipulate operation of transistor Q1214 to manipulate the gate voltage of LDFET Q2 228 so as to force moreor less current to flow across the drain current sense resistor R1 204.This may be particularly useful in over-temperature situations when itis desirable to have greater current flowing during “hot” operation.

Although improved, the circuits of FIGS. 2 and 3 suffer the disadvantageof drawing the same drain current Id 227 whether or not there is anyinput RF power 220 present. To solve this disadvantage, the amplifier400 of FIG. 4 includes an RF Level Detector, U2 434, which senses theinput RF power 420 and presents a corresponding DC voltage to an inputof a comparator, U1 446, which serves to turn on or shut off transistorQ2 based on a threshold reference voltage. In this manner amplifier 400avoids unnecessary current flow and resulting undesired effects. RFLevel Detector U2 434, in this example, is part number AD8314 assupplied by Analog Devices, Inc. and is a 50 dB dynamic rangeamplifier/conditioner used for transmitter power control and otherapplications. RF Level Detector U2 434 operates in the frequency rangeof 0.1 to 2.5 GHz and over a typical dynamic range of 50 dB. U2 434 isinternally AC-coupled and has high sensitivity for control at low signallevels.

RF IN 420 is connected to amplifier circuit 400 through capacitor C1 424(e.g., 12 pF) to input matching network 428 and through capacitor C3 422(e.g., 1.5 pF) to RFIn input of RF Level Detector 434. This simplifiedarrangement is particularly useful in narrow band applications. In onealternative, shown in FIG. 4A, RF IN 420 is connected to circuit 400through coupler 421, which taps off some of the power flowing in themain RF path for use in a lower power path such as to drive a pre-scaleror in this case a level detector 434. Note that for narrow bandapplications the simplified arrangement of FIG. 4, small value capacitorrather than coupler of FIG. 4A, has the advantage of significantlyreducing board space particularly as the operating frequency is reduced.For example at 2 GHz and a dielectric constant of 3.5 and a boardthickness of 20 mils, the coupler trace would be approximately 0.860″ inlength and 0.140″ in width. At 400 MHz, the trace length would increaseto 4.30″ with the same width. By comparison a 0402 capacitor occupies anarea of 0.04″×0.02″. The disadvantage of the simplified capacitorarrangement is that the amount of coupled energy becomes a function offrequency—Zc=1/(2π×F×C). Since the F term (frequency) is in thedenominator, the Zc decreases as frequency increases thereby increasingthe coupled energy. With consideration of this trade-off, eitherarrangement may be used in the circuits of FIGS. 4-13.

By way of coupler 422 (FIG. 4A), such as physical traces on a circuitboard, or through capacitor C3 422 (FIG. 4) a portion of the RF power isconnected to input 1 (RFin) of U2 434 which acts as an RF level detectorand presents a corresponding DC voltage to the input 3 of comparator U1446, for example a micro-power CMOS comparator available from NationalSemiconductor by part number LMC7221. Both U1 446 and U2 434 areconnected to a reference voltage +5Vdc 452. The input 2 of thecomparator U1 446 connects to potentiometer R9 444 which is adjusted tothe desired input threshold RF power. Below this threshold the circuitdraws only the current that is required by U1 446 and U2 434. Above thethreshold the circuit performs essentially as the circuit of FIG. 2. Theoutput 6 of U1 446 is connected to the base of transistor Q3 450, ageneral purpose transistor having part number 2N2222 or PN2222. Thecollector of Q3 450 is connected through resistor R4 408 to thepotentiometer R3 406 which is connected to the base of bipolartransistor 412, which is part of drain current control circuit 401,similar to drain current control circuit 201 discussed above. In thismanner, the combination of U1 446 and transistor Q3 450 control the base(B) to emitter (E) voltage (Vbe) of transistor Q1 412 below the setthreshold so as to shut off the flow from the collector of Q1 412. Thisremoves the voltage across resistor R6 430 thereby shutting off thedrain current, Id 417, in Q2 418.

The amplifier 500 of FIG. 5 includes an RF power leveling aspect. Inoperation, the output RF power 502 is output from Output MatchingNetwork 504 through coupler 506, which is connected via input 1 (RFin)of integrator U1 508. In this manner, U1 508 senses the RF OUT andoutputs at output 7 (Von) a corresponding DC voltage which is deliveredto input 3 of integrator U2 510 by way of circuit 512. U2 510 in thisexample is a low power operational amplifier having part number LM 7301as provided by National Semiconductor Corporation. By comparing thisvoltage with a preset voltage from potentiometer R9 514 the output ofthe integrator U2 510 will increase or decrease the voltage to resistorR4 516 to control operation of transistor Q1 518 so as to cause thedrain current Id 517 associated with transistor Q2 520 to increase ordecrease as required to maintain an essentially constant output RF power502.

The circuit of FIG. 6 is an optional substitute circuit for use in thecircuit of FIG. 5. FIG. 6 shows a digital to analog convertor 602 thatis used in place of R9 514 thereby permitting the RF output power to becontrolled by a computer command.

FIG. 7 illustrates an alternative amplifier 700 in which transistor Q2702 operates as a high power gain block—the gain being determined by theratio of the coupling coefficients of the output to input couplers704/706. The input and output RF levels 708/710 are measured by RF LevelDetectors U3 714 and U1 712 respectively. The outputs Von of RF LevelDetectors U1 712 and U3 714 are delivered, respectively, to inputs 3 and4 of integrator U2 716. The output 1 of U2 will in turn increase ordecrease the voltage to resistor R4 718 causing the drain current Id 703to increase or decrease as required cause the correct amount of currentto flow through Q2 702 to cause the two inputs to equalize and maintaina constant output RF power.

If, for example, the input and output couplers 704/706 have couplingcoefficients of 10 dB and 15 dB respectively, the overall circuit willexhibit a gain of 5 dB. One major advantage of this design over priordesigns is that unlike conventional gain blocks, the drain current Id703 varies according to the output power required to maintain a constantgain.

FIG. 8 is a schematic of an optional digital attenuator 800 placed inthe input 1 (RFin) of the input power detector 714 of FIG. 7 asindicated at nodes 801 and 802. With the optional digital attenuator,amplifier 700 still performs as a gain block but with the added featureof selectable gain and also retains the advantage of drawing only thatcurrent Id 703 required to maintain the desired gain. While the gain isbased in part on the coupling coefficients of couplers 704 and 706, thedigital attenuator allows for programmable or selectable gain to adjustthe actual gain achieved. In this example, attenuator 800 is part numberHRF-AT4521 as made by Honeywell International Inc. and is a 31.0 dB,DC−2.5 GHz, DC-coupled, 50 ohm impedance 5 Bit Serial DigitalAttenuator. In operation, a serial digital word such as received from asystem control unit, is input to a register contained within thisattenuator. This stored digital word then sets the attenuation ratiountil a new word is received or DC power is cycled.

With reference to FIG. 9, a recent transistor technology is that ofGallium Nitride Field Effect Transistor (GaNFET), such as transistor Q1902, which has among its advantages higher operating voltage, highertemperature operation, higher efficiency, and higher frequency operationthan LDFETs. Both the GaNFET Q1 of FIG. 9 and the LDFET Q2 of FIG. 2 arepositive sense with respect to the gate voltage in that a more positive(or less negative in the case of GaNFET) voltage on the gate results inhigher drain current. While a typical gate voltage on a LDFET may be inthe +1 to +2 VDC range, the gate on a GaNFET will normally operate inthe −2 to −1 VDC range. This applies equally to GaAs FETs and JFETS forthat matter. GaNFET transistors have similar gate drift problems asLDFETs, but as stated above they have the further complication of anegative gate voltage requirement. In many situations, such as inairborne applications, it is unusual to have a negative voltageavailable. To satisfy the negative voltage requirement, prior circuitssuch as FIG. 9 used a negative voltage driver IC such as the MaximSemiconductor MAX881 (U1) 902. The output 3 (NegOut) of this device ispresented to diodes CR1-4 904 and potentiometer R1 906 connected inseries. Much like the LDFET prior art as discussed above, prior GaNFETtransistor circuits used a similar approach of adding or removing diodesto accomplish temperature compensation for the threshold voltage on anopen loop basis. Again, one problem with this solution is that thetemperature compensation procedure is very labor and equipmentintensive. Another problem is that U1 902 includes a square waveoscillator which causes spurious responses on the output of GaNFET powertransistor Q1 908. In this example Q1 908 is a 25 W, RF Power GaNFETwith part number CGH40025F as manufactured by Cree, Inc. There are manysuch devices available along with publicly available application notesexpressing techniques concerning how to minimize such spurs.

With reference to FIGS. 10A and 10B, one of the advantages of the GaNFETamplifier 1000 over prior circuits is that it does not attempt tominimize the spurious responses caused by the negative voltage deriverIC by better decoupling or improved grounding or any of the othertechniques shared in the application notes. Rather, amplifier 1000eliminates the problem entirely by shutting down the oscillator 1040once it is no longer needed. In general, the oscillator of negativevoltage driver U1 1040 is only needed momentarily as a “starter” for afew milliseconds, i.e., long enough for transistor Q2 1010 to come tolife. To avoid the undesired effect of the spurious oscillator signal ofthe negative voltage driver U1 1040, the invention shuts down the driverafter start up and relies on the RF output transistor Q2 itself tomaintain operating conditions. The RF on the drain of Q2 1010 generatesand provides the negative supply for the transistor. Once the transistorQ2 1010 “comes to life” and self supplies the negative gate voltage thenthe negative voltage driver 1040 shuts down and in this manner theamplifier 1000 removes the spurious oscillator signal of the driver1040.

Amplifier 1000 is connected to supply voltage 1002 (+20 VDC) andreceives RF IN 1004 which is connected to the gate of transistor Q2through capacitor 1006 and input matching network 1008. In this example,transistor Q2 1010 is a GaNFET having part number GCH40025F, asdescribed above. Referring to FIG. 10B, negative voltage driver U1 1040as connected generates approximately −5.0 VDC at output 3 (NegOut). Thisvoltage then passes through Schottkey diode CR3 having part numberCDBU0130L, such as made by Discrete Semiconductor Products, to present−4.6 VDC to the negative supply (input 2) of operational amplifier U21044. U2 1044, as connected at node 1002, then presents a regulated −4.5VDC to resistor R6 1024. The key is to configure the circuit so that thelevel is more negative than the level required to shut down Q2 1010. Thegate bias circuit 1012 includes drain current sensing resistor R1 1014,bipolar PNP transistor 1016 (connected to potentiometer R3 1020), andresistor R5 1018 and operates much as does the LDFET gate bias circuit201 described above except in the case of the GaNFET gate bias circuitthe voltage increases to the value required by starting from a negativepotential rather than from ground (as in the case of the LDFET biascircuit 201). In both cases the sense is positive, i.e., increasingvoltage on the transistor gate, in this case of transistor Q2 1010,causes an increase in drain current, in this case drain current Id 1011.

The shutoff function is accomplished by a coupler 1046 which pulls afraction of the RF power present at the drain of Q2 1010 and presentsthis power to the input of the negative voltage deriver 1048, comprisedof capacitors C3 and C4 and diode pair CR2 (part number 2822), operatingat the carrier frequency. This detected negative voltage is thenpresented to the negative supply (pin 2) of U2 1044 through CR4 1050. Inaddition, this negative voltage is connected to resistor R12 which inconjunction with resistor R11 are connected to pin 5 (SHDN) of U1 1040and causes the oscillator section of U1 to shut down. U1 1040 then ineffect acts as a starter, i.e., it operates only long enough for theamplifier U2 1044 to begin delivering RF power and then automaticallyshuts off thereby eliminating spurs caused by its internal oscillator.

The remainder of the circuitry in FIG. 10 acts similarly to that of FIG.2 in that as the gate voltage is increased (less negative), the currentin the drain of Q2 increases until an equilibrium point is reached.

The various improvements described above and shown in relation to FIGS.3-8 as relating to the LDFET implementation of the invention also may beapplied in the context of GaNFET implementation of FIGS. 10A-10B. Theoptional temperature compensation circuit 300 described above forincorporation into the amplifier 200 of FIG. 2 (as shown at connectionnodes 301 and 302) may also be included to the amplifier 1000 at thesame location in the circuit as shown in FIG. 10A. Also, as describedabove in the LDFET implementation of FIG. 4, the circuit of FIG. 11 mayoptionally be included with the amplifier 1000 of FIG. 10A to overcomethe disadvantage of drawing the same drain current Id 1011 whether ornot there is any input RF power 1004 present. Connection points areshown on FIG. 10A with hatched lines. As with FIG. 4, the circuit ofFIG. 11 includes an RF Level Detector, integrator U2 1134, that sensesthe input RF power 1004 and a comparator, U3 1146, that serves to shutoff the drain current Id 1011 of Q2 1010 of FIG. 10A based on athreshold reference voltage. In this manner the amplifier 1000 avoidsunnecessary current flow and resulting undesired effects.

Also, as described above in the LDFET implementation of FIG. 5, thecircuit of FIG. 12 may optionally be included with the amplifier 1000 ofFIGS. 10A/10B. As described for the amplifier 500 of FIG. 5, the circuitof FIG. 12 includes an RF power leveler aspect. In operation, the outputRF power 1052 is output from the Output Matching Network through coupler1046, which is connected via input 1 (RFin) of integrator U3 1208. Inthis manner, U3 1208 senses the RF OUT 1052 and outputs at output 7(Von) a corresponding DC voltage which is delivered to input 3 ofintegrator U4 1210 by way of circuit R12, R13, and C4. By comparing thisvoltage with a preset voltage from potentiometer R9 1214 the output ofthe integrator U4 1210 will increase or decrease the voltage to resistorR4 1026 of FIG. 10A to control operation of transistor Q1 1016 so as tocause the drain current Id 1011 associated with transistor Q2 1010 toincrease or decrease as required to maintain an essentially constantoutput RF power 1052.

As shown by the hatched lines on FIG. 12, the circuit of FIG. 6, asdescribed above, is an optional substitute circuit for use in thecircuit of FIG. 12 and amplifier 1000 of FIGS. 10A/10B. FIG. 6 shows adigital to analog convertor 602 that is used in place of R9 1214 therebypermitting the RF output power to be controlled by a computer command.

In addition, the alternative circuit of FIG. 7, as implemented inconjunction with LDFET amplifier 700, may be employed in conjunctionwith GaNFET-based amplifier 1000 of FIGS. 10A/10B. The operation oftransistor Q2 1010 of FIG. 10A as a high power gain block is similar tothe operation of Q2 702 of FIG. 7 described above. Also, the optionaldigital attenuator 800 of FIG. 8 may be included in the circuit foroperation with the GaNFET amplifier 1000 of FIG. 10A in the same manneras described above in the context of LDFET-based amplifier 700 of FIG.7. With the optional digital attenuator, amplifier 1000 still performsas a gain block but with the added feature of selectable gain and alsoretains the advantage of drawing only that current Id 1011 required tomaintain the desired gain.

These circuits thus far described, including specifically FIGS. 10-12related to GaNFET embodiments, also operate with other negative voltagegate devices such as N-Channel Junction Field Effect Transistors (JFET)and Gallium Arsenide Field Effect Transistors (GaAsFETS).

The present invention is intended for many uses and applicationsincluding in design and manufacture of airborne and ground-basedtelemetry equipment. Telemetry systems comprise transmitters, receivers,and power amplifiers in a wide variety of frequency ranges. Althoughmany aspects of the various embodiments of the invention are describedin the analog domain it should be understood that this is for purposesof explaining the invention and that the invention may be used inconjunction with digital processing devices and techniques including theuse of microprocessors, memory, etc.

FIGS. 13A/13B illustrate an alternative embodiment that corresponds tothe GaNFET-based amplifier of FIGS. 10A/10B. With reference to FIG. 13B,as described in detail above in the context of FIG. 10B the output ofthe dual diode-based negative voltage deriver 1048 provides a voltagesource for the bias circuit. In addition, the output of the negativevoltage deriver 1048 provides a crude measure of the output power. Byadding resistors 1313 (R13) and 1314 (R14), and transistor 1310 (Q3) tothe previous circuit of FIGS. 10A/10B, drain current 1011 will vary tosupply only the current required to deliver that output power. In thisregard, this circuit will perform similarly to a Class H amplifierexcept drain current is modulated rather that drain voltage. Class Hamplifiers operate with infinitely variable supply rails. This is doneby modulating the supply rails so that the rails are only a few voltslarger than the output signal at any given time. Switching regulatorpower supplies are commonly used to create the tracking rails. Theproblem with Class H amplifiers is that switching regulators havenotoriously low bandwidth, usually sufficient only for audiofrequencies. The circuit of the invention on the other hand can operatewell into the MHz range and is a far simpler and less expensiveimplementation.

In accordance with this alternative design, the exemplary 10K ohmresistor 1026 of FIG. 10A is replaced with a 100 K ohm resistor 1326 inthe circuit of FIG. 13A. In operation, a small current through resistor1326 (R4) results in a residual drain current in transistor 1010 (Q2)sufficient only to keep it “alive” or “active” and capable ofamplifying. As a RF signal is applied to the RF input 1004 (FIG. 13A),this signal is amplified and a negative voltage is generated at theoutput of the dual diode detector CR2. Transistor 1310 (Q3) then beginsconducting thereby lowering the voltage at the base of transistor 1016(Q1), causing more drain current to flow through transistor 1010 (Q2).Note that this drain current will only be sufficient to generate theoutput power as determined by the level on input power and the gain ofQ2. Resistor 1314 R14 prevents Q2 from excessive drain current bylimiting the extent to which the base of Q1 can be lowered in voltagedue to Q3 entering saturation should the dual diode detector/resistor1313 R13 combination call for excessive current. This circuit therebyprotects RF transistor from damage.

With continuing reference to FIG. 13B, the negative regulated voltage atthe output of amplifier 1044 (U2) can optionally be provided tosubsequent amplifier stages so that the circuitry of FIG. 13B would notneed to be repeated resulting in significant efficiencies. As shown inFIG. 13B, the base of transistor Q3 may be connected to ground.Alternatively, and as shown in FIG. 13C, the base of Q3 may be connectedto the circuit comprising +5 Vdc through 10 K ohm resistor 1315 (R15)diode 1316 (CR5) to ground.

Now with reference to FIG. 14, in one embodiment the present inventionprovides an improved N-Channel depletion device-based amplifier 1400with a novel gate bias circuit 1401 (largely as discussed hereinabove inconnection with other exemplary embodiments) and a novel sequencercircuit as shown in hatched line 1402, which is particularly useful inRF receiver/transceiver applications. Bias sequencer circuit 1402includes a P-channel enhancement mode MOSFET pass transistor 1404 foruse in operation of Q3 1406—an N-channel depletion mode device—inconjunction with drain choke inductor L1 1408. Amplifier 1400 alsoincludes sampling circuit 1412 and negative voltage deriver circuit1414. In this example, Q3 1406 is a device available from Cree, Inc. andhaving product number CGH40006P, which is a gallium nitride (GaN) highelectron mobility transistor (HEMT) offering 28 VDC and up to 6 GHzoperation. Gallium Nitride (GaN) HEMTs (High Electron MobilityTransistors) or GaN FETs or GaN MMICs (Monolithic, Microwave IntegratedCircuit) (referred to herein collectively and interchangeably) are arelatively new generation of RF power transistor technology that offersthe operating characteristics of higher power, higher efficiency andwider bandwidth when compared with GaAs- and Si-based technologies,e.g., LDMOS FETs. GaN HEMTs are depletion mode devices, meaning anegative gate voltage and gate-drain bias sequencing is required forproper operation. With any N-Channel depletion device, such as GaAs FET,GaN FET, or N-channel silicon junction FET, it is essential that thenegative gate voltage arrives before the drain voltage. Otherwise thedrain to source resistance is a very low value which will essentiallyshort out the input power and likely cause damage to several circuitcomponents including the depletion device.

The CGH40006, operating from a 28 volt rail, provides a general purpose,broadband solution to a variety of RF and microwave applications. GaNHEMTs provide high efficiency, high gain and wide bandwidth capabilitiesmaking them attractive for linear and compressed amplifier circuits in avariety of applications, including aviation, communication (includingtelecommunications), weather RF. Moreover, material characteristicsassociated with wide-bandgap III-nitride materials such as AlGaN/GaNHEMTs make them attractive for use in integrated circuits to performreliably in operations at high temperature not readily possible forsilicon- or GaAs-based devices. These high-temperature digital circuitsprovide improved and enabling technology for many applications includingintelligent control and sensing for use in automotive, aviation,chemical reactor, and oil exploration systems.

With continuing reference to FIG. 14, sequencer 1402 includes U1 1410,which is a low dropout positive voltage regulator that follows the inputvoltage within millivolts up until the desired output voltage isreached—in this case+5 VDC. Q1 1404 is an enhancement mode P channelMOSFET with a minimum Vgs threshold of between 2 VDC and a maximum of 4VDC. Q3 1406 is a GaN FET device with a pinch off voltage of −3.0 VDCtypically. Pinch-off refers to the voltage Vds that counteracts theopening of the n-channel (NMOS), at the drain end. Pinch off voltage isthe minimum ground-to-source voltage for which drain current will becomezero and if applied between drain and source then drain current startssaturating. The MOSFET or MESFET saturates (pinches off) when Vds isgreater or equal than/to Vgs−Vtn. Negative Voltage deriver circuit 1414includes U2 1415, which is a negative voltage deriver, such as anoscillator, that also is a low dropout device—i.e., its output voltage(at pin 5) follows its input voltage within millivolts, though invertedand therefore negative.

For example at a circuit input voltage of +5.0 VDC, U1 output will beapproximately +4.9 VDC and the output of U2 1415 will be approximately−4.8 VDC. The negative rail of U3 1417 will then be about −4.1 VDCassuming a diode drop across CR5 1416 of 0.7 VDC. As the input voltageis increased from 0 VDC, the output of U1 1410 closely tracks the inputuntil the specified regulator output voltage of +5 VDC is reached.

Assuming a minimum threshold voltage of 2 VDC for Q1 1404, this devicewill therefore not begin turning on until an input voltage on the +Vinline reaches 5 VDC+Vth=+7 VDC and at 7 VDC no current through drainchoke inductor L1 1408. A threshold voltage of 4 VDC would increase thisto +9 VDC. It is imperative that there is minimal current through Q11404 while in the linear state between fully off and fully on. Thisresults in the minimum dissipation in this device which permitsminiaturization, which is an important advantage to the invention. Inthe event it may be desirable to increase the apparent threshold voltageof Q1, a resistor can be placed between the gate and source as shown(R1) 1418.

With an input voltage of +7 VDC on the +Vin input line and assuming nodrain current through Q3 1406, and no voltage at R3, Q2 1420 will besaturated since in this situation there is actually more base currentthan collector current. The Q2 1420 collector voltage will be at about+6.8 VDC which through voltage divider R6 (15 Kohm)/R7 (1 Kohm) willresult in a voltage at Pin #3 of op-amp U3 of +0.425 VDC and a voltageat input #4 of U3 of 4.575 VDC. This would result in an output voltageof the op amp of −4.150 VDC which when applied to Q3 1406 through L21422 places this device well in to the pinch-off region—therefore nodrain current. Voltage divider R6/R7 insures that Q1 does not have powerdissipated through it (thus no need for heat sink). Even in a barely oncondition current is only 1-2 milliamps. The voltage divider also setsthe maximum voltage out of op amp U3 at −1.5 V and never positive as asafeguard for Q3 1406. This circuit ensures that Q1 is well on beforecurrent flows through the drain of Q3. In this circuit Q1 may be a smalldevice avoiding the need for a heat sink thus resulting in desirableminiaturization. In addition, the circuit avoids the need for a couplerthus resulting in further miniaturization.

Drain current setting resistor R3 1407 is a sense resistor that sets thecurrent through Q2. In addition, during fabrication, maintenance orrepair, when Q3 is not part of the circuit leads may be provided toconnect the circuit to various GaN/GaAs devices. Leads across R3 may beused to set current depending on the particular Q3 device used. In theexample of FIG. 14, Q3 is a GaN HEMT device available from Cree, Inc. ofDurham, N.C. (www.cree.com) having part number CHP40006P. Parts are alsoavailable from Gain Microwave (www.gainmicrowave.com) and from NitronexCorporation (www.nitronex.com).

Working backwards through the circuit and assuming a threshold pinch-offvoltage of −3.00 VDC for Q3 1406, it can be shown that this devicestarts conducting at an input voltage of +16.2 VDC on the +Vin line.From 7 VDC to 16.2 VDC there is no current through Q1. This shows thatQ1 1404 is well into its low impedance state before there is appreciablecurrent through it—again this being desired for minimal dissipation inthis device.

Normal operation of Q3 1404 occurs with the gate at approximately −2.0VDC although this varies somewhat from device to device, temperature,frequency, and time. This corresponds to an input voltage on the +Vinline of approximately +24 VDC. For any voltage above+24 VDC the circuitperforms as the others described in this application. Note that anotherdesirable feature of this circuit is that at input voltage of +28 VDC,the highest voltage that can be applied to the gate of Q3 is −1.5 VDC.It is important that this voltage never reaches a positive value or Q3would likely be damaged or destroyed.

Operation of the negative deriving circuit 1414, U2 1415 and associatedcomponents, operate similarly to the previously described circuitsinvolving depletion mode devices with the exception that the output RFpower is sampled via sampling circuit 1412, comprising C4 and R8, ratherthan a coupler as described elsewhere with respect to other circuits,e.g., coupler 506 of FIG. 5. This also contributes to the advantage ofcircuit miniaturization.

The above circuit is described in the context of a first scenario inwhich +V_(in) is, for example, a battery source that slowly rises atturn on from 0 to 28 VDC. In an alternative scenario, an instantaneous28 VDC source may be switched on or otherwise supplied. In thisscenario, capacitor C1 1419 slows down to allow the negative voltage tocome up. Value for C1 in this example of FIG. 14 is 1 μF but may bederived from time needed to allow negative voltage deriver circuit tocome up. In cases in which the +Vin is applied instantaneously, C1 1419causes a delay in the turn on of Q1 1404 so that the negative generator1414 will have time to power up and generate the negative gate voltagebefore the drain voltage arrives. Upon shut down, CR1 1421 provides arapid discharge path for C1 1419 so that Q1 1404 shuts off before thenegative gate voltage collapses. This prevents Q3 1406 from having drainvoltage without a negative gate voltage which could result in damage tothis device during shut down. If the output of U3 1417 goes positive,the Q3 would be damaged or destroyed as the GaN HEMT must always benegative. By using these techniques, the invention in essence“domesticates” Q3 1406.

Optionally, an automatic drain current adjustment circuit may be addedto the circuit of FIG. 14 connecting between node 1424 and the input tobase of Q2. For example, the emitter of an NPN bipolar transistor (notshown) may be connected through a resistor to node 1424 with the base ofthe BJT connected to a +5 VDC source (resistor-diode-ground arrangement)the collector of the BJT is connected through a resistor to the inputcircuit to the base of Q2 1420. In this manner, drain current may be setas a function of input power (sensed). For example, with low RFin andlow RF out the node 1424 is not very negative and therefore can lowerdrain current.

In addition, features as described above in relation to other circuitsmay be used on connection with the circuit of FIG. 14 and those thatfollow. For instance, RF level detectors (712/714), adjusting means toset current (716), failure detection, damage prevention, etc. Likewiseuse of features described in connection with FIGS. 14-19 may be used inconnection with other circuits discussed above. For example, anopto-coupler may be used as the negative voltage generator U2 1415 ofFIG. 14 and for the negative voltage generator U1 1040 of FIG. 10B. Anexemplary opto-coupler is the opto-isolated photovoltaic Iso-gate MOSFETDriver available from Dionics, Inc. (www.dionics-usa.com) with partnumber DIG-12-06-250M.

Also, in comparing the circuit of FIG. 14 (and those that follow) withthat of FIG. 10A/B, we see that the circuit of FIG. 14 has the advantageof going directly into the gate of Q3 1406 (FIG. 14) rather than throughresistor R6 1024 and into Q2 1010 (FIG. 10A). For example, R6 1024 makesthe circuit too soft in that much over 1 milliamp from gate overwhelmsand makes it less negative at R6 and this may lead to loss of control ofcircuit. Preferably the circuit operates at −2.0 V at 1 milliamp. Forexample, in one application the standard is around 2 milliamps and withthe circuit of FIG. 14 operation works for Q3 1406 up to 5 milliampsgate current. Also, the circuit of FIG. 14 provides greaterminiaturization in using Q1 1404, which does not need a heat sink andreplaces external sequencer not shown with respect to the circuits shownin FIGS. 2-13. Also, the circuit of FIG. 14 and those that follow do notneed two sequenced supplies for gate and drain but rather a singlesupply Vin for input into the internal sequencer, e.g., 1402.

Now referring to FIG. 15, in this embodiment the present inventionprovides an improved N-Channel depletion device-based amplifier 1500with a novel gate bias circuit 1501 (largely as discussed hereinabove inconnection with other exemplary embodiments) and a novel sequencercircuit as shown in hatched line 1502. The purpose of the sequencer 1502is to ensure that the GaN device Q4 1506 is in pinch-off before thedrain voltage is applied. If the drain voltage is applied before thegate voltage, Q4 1506 essentially acts as a short circuit and either itor the power supply or both will be destroyed. Since GaN transistordevices are N-Channel enhancement mode devices, this means that the gatevoltage (Vgs) must be at the most negative specified voltage thresholdor less (more negative). For this example, a negative gate voltage of−3.8 Vdc or more negative is required to assure pinch-off for Q4 1506.Bias sequencer circuit 1502 includes Q1 1504, which is an enhancementmode P-Channel MOSFET with a minimum specified threshold voltage of−1Vgs and a maximum of −3Vgs. U1 1510 is a low drop-out regulator with adrop out voltage <0.5 Vdc.

As Vin increases from 0 Vdc to +5 Vdc, the output of the low drop outregulator U1 1510 tracks the input within 0.5 Vdc. Therefore the Vgs ofQ1 1504 is always <−0.5 Vdc so Q1 is off during that period. When Vinreaches+7Vdc, Q1 reaches its lower specified threshold voltage of −1 Vgsdue to the voltage dividing action of R1 and R5.

As Vin further increases, Q1 1504 will begin conducting. When the drainof Q1 reaches approximately +1.4 Vdc, Q2 1505 turns on and pulls thegate of Q1 more negative with respect to its source. Thus Q2 1505provides regenerative feedback to Q1 causing it to operate in a lowerRds region. This is desirable so as to minimize the dissipation in Q1.

At the upper Vgs threshold specification of −3Vgs, Q1 will begin turningon at a Vin of +11 Vdc. Similar to above when Q1 begins conducting andits drain reaches +1.4 Vdc, it will be turn on harder due to theregenerative feedback action of Q2 1505.

One very desirable feature of this circuit is that the switchingtransistor Q1 1504 conducts essentially no current during its transitionbetween the On and Off states so as to minimize dissipation in thisdevice as is required for circuit miniaturization. Therefore Q4 1506must be in pinch-off during this transition—which we will now prove.

At Vin of 11 Vdc (the upper limit of threshold voltage for Q1) thenon-inverting input (pin #3) to op amp U3 1517 will be at +1.19 Vdc. Theoutput (pin #1) of U3 1517 will then attempt to go to −4.98 Vdc but willbe limited by the negative rail to actually go to approximately −4.4Vdc. This voltage is then applied to the gate of Q4 1506 through L3.Since the CGH4000P has a maximum pinch-off specification of −3.8 VDC, itwill be well into pinch-off. At the lower threshold voltagespecification of Q1 of −1 Vgs, the output of U2 will attempt to go to aneven more negative voltage but will be again be limited to approximately−4.4 Vdc so Q4 will be well into in pinch-off in either case.

Normal operation of Q4 occurs in the −2 Vgs region which will requireVin=+21.5 Vdc which is well below the recommended +28 Vdd for thedevice. This means that once Vin exceeds +21.5 Vdc, the sequencer fallsout of the circuit and will in no way interfere with the correctoperation of Q4.

For the case in which Vin is applied instantly, C1 1519 prevents Q1 1504from turning on until U2 1515 and associated circuitry is able toprovide the required negative voltage. With the values shown, U2 1515reaches −5 Vdc in approximately two milliseconds whereas C1 1519 delaysthe turn on of Q1 for about 20 milliseconds (mS).

During power shutdown it is imperative that Q1 shuts off before thenegative gate bias to Q4 is removed. This is accomplished by CR1 1521which provides a low impedance path to C1 1519 so that Vgs of Q1 1504will be removed within microseconds whereas C8 will hold the negativegate voltage to Q4 for approximately five mS.

R24 and C12 offer an amplitude modulation input port 1526 which operatesas follows. Q4 must first be driven well into saturation—by increasingthe input RF drive by 6 dB (for example) beyond the one dB compressionpoint (Pldb). Further, the drain current should be set well below itsmaximum recommended value—for example 50%. While operating in thisconfiguration, output RF power will now be determined by the draincurrent rather than by the input RF drive as is the usual case foramplitude modulated amplifiers. As the voltage to the modulation inputport is increased (more positive), Vgs increases (less negative), thedrain current then increases, and as a direct result the output RF poweralso increases. In this manner the amplitude of the output RF power ismodulated by the signal at the modulation input port.

This circuit will find significant application in advanced digitalmodulation applications which depend upon amplitude modulation. Theadvantage of this circuit is that it always operates in saturation asopposed to conventional AM amplifiers which operate in Class-A. Theaccompanying increase in efficiency will be very substantial. Forexample Class-A amplifiers have a maximum theoretical efficiency of 50%whereas saturated amplifiers can exceed 80%.

Now referring to FIG. 16, a circuit 1600 is shown that is in essence thecircuit 1500 of FIG. 15 modified to replace the U2-based negativevoltage deriver circuit 1514 with an Optically Coupled NegativeGenerator circuit 1614. Circuit 1600 operates much as circuit 1500 withthe difference that an optically coupled isolated voltage generator U71615 is substituted for the negative voltage generating oscillator 1515.With the positive output pins (5 & 7) of U7 1615 grounded, a negativevoltage is available on Pins 6 & 8. Since this is a spur-free generator,there is no need to shut it off as in the previous circuits. Thedisadvantage is that the generator's output is in the microamp range sothis circuit will operate only with GaN devices with very low gatecurrent.

Now referring to FIGS. 17A and 17B, showing connecting points 17B1-17B3,in this embodiment the present invention provides an improved N-Channeldepletion device-based amplifier 1700 with a novel gate bias circuit1701 (as discussed hereinabove in connection with other exemplaryembodiments) and a novel sequencer circuit 1702 along with an AdaptiveCurrent Control Circuit. The Adaptive Current Control Circuit includesU4 1703, which along with its associated components adds several usefulfeatures to the control circuit. U4 1703 measures the input RF power andoutputs a voltage which is approximately linear over its input powerrange of −5 dBm to +5 dBm. The output of U4 (pin #5) goes to an emitterfollower Q5 1704 the collector of which acts as a variable current sink.As the input power is increased, the output voltage of U4 1703 increaseswhich in turn causes Q5 1704 to sink more current. As this current isdrawn from the junction of R7, R8, R15 and the base of Q3 1705, thevoltage across the sense resistor R6 is increased causing the draincurrent of Q4 1706 to increase. The numerous applications for thecircuit, including the Adaptive Current Control Circuit which lowersdrain current for greater efficiency, includes CDMA (code divisionmultiple access) based and other RF and radio communicationtechnologies.

Note that as resistors R8, R15, R16, R17 are adjusted, an infinitenumber of RF input power versus drain current curves are made possible.See FIGS. 20A and 20B sample curves A through G. Curve A is the constantcurrent configuration which operates as the circuits of the main patent.Curve B is especially interesting in that it causes the circuit toperform as an adaptable Class-A device—i.e. the drain current of Q4 1706changes as a function of the input RF power. The overall circuittherefore performs with the efficiency of Class-AB operation but withthe linearity of Class-A. Traditional Class-A circuits draw the samecurrent independently of input RF power resulting in much worseefficiency at low input levels.

Curve C would result in some residual current to keep Q4 “alive” withlow or no input signals. This is often desirable as the GaN devices cango unstable with low drain current. Curves D and F are related, thedifference being the steepness of the Pin versus Id slope. Curve F isessentially a switch turning on Q4 1706 only when input RF is present.Curve E is a combination of Curves C and D.

Now referring to the circuit of FIGS. 18A and 18B, showing connectingpoints 18B1-18B4, the negative voltage at the junction of CR4, CR5, C8,and U3 1802 (FIG. 18B) is a rough measure of the output RF power. Whenthis voltage is applied to R19 1804 (FIG. 18A) a current is pulled fromthe emitter of Q6 1805 and ultimately from the junction 1807 of R7, R18,R8, and Q3. As the output power is increased, more current is drawn fromthe junction which in turn causes Id of Q4 1806 to increase. Note thatthis circuit requires a residual current as in Curve E set by R8 lestthe circuit will not start. R18 sets the maximum Id since at some pointQ6 will saturate and no more current will be drawn from the abovejunction regardless of the level of output power.

Now referring to the circuit of FIGS. 19A and 19B, showing connectingpoints 19B1-19B3, circuit 1900 is a less sophisticated version of theAdaptive Current Control Circuit of FIGS. 17A and 17B but is also lesscostly and occupies less space leading to further miniaturization.Unlike the circuit of FIGS. 18A and 18B, circuit 1900 does not require aresidual current thereby permitting it to act as a switch—turning on Idonly when input RF power is present. Curves A through G of FIGS. 20A and20B can be approximated with this circuit but with somewhat lessaccuracy than with the circuit of FIGS. 17A and 17B. CR7 1902 and C101903 (FIG. 19A) detect the input RF level and convert this to a negativevoltage which is applied to R21 1904. This then draws current from thejunction 1905 of R7, R8, R22, and Q3 acting through Q7. Similar toabove, R22 1906 sets the maximum current that can be drawn from thisjunction 1905 effectively setting the maximum Id. R20 1907 and CR6 1908bias the base of Q7 to approximately +0.7 VDC which puts the emitter ofQ7 very close to 0 VDC so that the detector can operate correctly at lowlevels of input RF power. CR6 1908 also temperature compensates the Vbeof Q7.

The present invention is not to be limited in scope by the specificembodiments described herein, It is fully contemplated that othervarious embodiments of and modifications to the present invention, inaddition to those described herein, will become apparent to those ofordinary skill in the art from the foregoing description andaccompanying drawings. Thus, such other embodiments and modificationsare intended to fall within the scope of the following appended claims.Further, although the present invention has been described herein in thecontext of particular embodiments and implementations and applicationsand in particular environments, those of ordinary skill in the art willappreciate that its usefulness is not limited thereto and that thepresent invention can be beneficially applied in any number of ways andenvironments for any number of purposes. Accordingly, the claims setforth below should be construed in view of the full breadth and spiritof the present invention as disclosed herein.

What is claimed is:
 1. An RF amplifier circuit comprising: FET forreceiving a RF input signal and generating an amplified RF outputsignal, the FET having a gate, drain, and source; control circuit,connected to the gate and drain of the FET, for controlling the currentat the drain and comprising a means for biasing and variablycompensating drift in the gate threshold voltage required to set thequiescent drain current, the control circuit being adapted to maintainan essentially constant current at the drain in connection with awake-up transition; RF input detecting means, operably connected to theRF input signal, for detecting the power level of the RF input signaland supplying a first DC voltage representative of the detected RF inputpower level; means for producing a variable reference voltage; comparingmeans, operably connected to the RF input detecting means and thevariable reference voltage, for comparing the supplied first DC voltageto the variable reference voltage; and switching means, operablyconnected to the comparing means, for turning off the FET based at leastin part on a comparison of the supplied first DC voltage and thevariable reference voltage.
 2. The circuit of claim 1 further comprisinga small value capacitor operably connected to the RF input and the RFinput detecting means.
 3. The circuit of claim 1 further comprising: anRF output detecting means, operably connected to the RF output signal ofthe circuit, for detecting the power level of the RF output signal andsupplying a second DC voltage representative of the detected outputpower level; and an adjusting means, connected to the RF input detectingmeans, the RF output detecting means and the control circuit, forautomatically adjusting the drain current based at least in part on acomparison of the second supplied DC voltage and the first supplied DCvoltage by an amount necessary to maintain an essentially constant gain.4. The circuit of claim 1 wherein the FET is an N-Channel depletion modedevice.
 5. The circuit of claim 4 further comprising a sequencer adaptedto maintain the FET in pinch-off condition before the drain voltage isapplied to avoid the FET acting as a short circuit.
 6. The circuit ofclaim 5 wherein the bias sequencer comprises a low drop out voltageregulator.
 7. The circuit of claim 5 wherein the sequencer comprises aP-channel MOSFET.
 8. The circuit of claim 1 further comprising anadaptive current control circuit adapted to measure input RF power andto output a signal representing the input RF power, whereby duringoperation of the circuit an increase in input RF power causes the draincurrent of the FET to increase and a decrease in input RF power causesthe drain current of the FET to decrease.
 9. The circuit of claim 1further comprising an adaptive current control circuit adapted toreceive a signal representing output RF power, whereby during operationof the RF amplifier circuit an increase in output RF power causes thedrain current of the FET to increase and a decrease in output RF powercauses the drain current of the FET to decrease.
 10. The circuit ofclaim 1 further comprising a means for switching on and off the draincurrent of the FET, whereby drain current is permitted only when inputRF power is sensed.
 11. The circuit of claim 1 wherein the first DCvoltage is presented to an input of a comparator, which serves to turnon or shut off a transistor based on a threshold reference voltage so asto avoid unnecessary current flow.
 12. The circuit of claim 1 whereinthe RF input level detector operates in the frequency range of 0.1 to2.5 GHz and over a typical dynamic range of 50 dB.
 13. The circuit ofclaim 1 wherein the RF input level detector is internally AC-coupled andhas high sensitivity for control at low signal levels.
 14. A methodcomprising: receiving, by a FET, a RF input signal and generating anamplified RF output signal, the FET having a gate, drain, and source;controlling the current at the drain by biasing and variablycompensating drift in the gate threshold voltage required to set thequiescent drain current; maintaining an essentially constant current atthe drain in connection with a wake-up transition; detecting the powerlevel of the RF input signal and supplying a DC voltage representativeof the detected power level; producing a variable reference voltage;comparing the supplied DC voltage to the variable reference voltage; andturning off the FET based at least in part on the comparing step.
 15. AnRF amplifier circuit comprising: a FET for receiving a RF input signaland generating an amplified RF output signal, the FET having a gate,drain, and source; a control circuit, connected to the gate and drain ofthe FET, for controlling the current at the drain; a bias circuitcomprising a means for biasing and variably compensating drift in thegate threshold voltage required to set the quiescent drain current, thebias circuit being connected to the control circuit and controllingoperation of the control circuit to maintain constant current at thedrain during a wake-up transition; means for detecting operationalconditions and for altering drain current subsequent to wake-uptransition to compensate for detected operational conditions; and meansfor selectively adjusting drain current throughout RF amplifieroperation independently of temperature change.
 16. The RF amplifiercircuit of claim 15 wherein the FET is an N-Channel depletion modedevice.
 17. The RF amplifier circuit of claim 16 further comprising abias sequencer adapted to maintain the FET in pinch-off condition beforethe drain voltage is applied to avoid the FET acting as a short circuit.18. The RF amplifier circuit of claim 17 wherein the bias sequencercomprises a low drop out voltage regulator.
 19. The RF amplifier circuitof claim 17 wherein the bias sequencer comprises a P-channel MOSFET. 20.The RF amplifier circuit of claim 15 further comprising an adaptivecurrent control circuit adapted to measure input RF power and to outputa signal representing the input RF power, whereby during operation ofthe RF amplifier circuit an increase in input RF power causes the draincurrent of the FET to increase and a decrease in input RF power causesthe drain current of the FET to decrease.
 21. The RF amplifier circuitof claim 15 further comprising an adaptive current control circuitadapted to receive a signal representing output RF power, whereby duringoperation of the RF amplifier circuit an increase in output RF powercauses the drain current of the FET to increase and a decrease in outputRF power causes the drain current of the FET to decrease.
 22. The RFamplifier circuit of claim 15 further comprising a means for switchingon and off the drain current of the FET, whereby drain current ispermitted only when input RF power is sensed.
 23. The RF amplifiercircuit of claim 15 further comprising: detecting means, operablyconnected to the RF input signal, for detecting the power level of theRF input signal and supplying a DC voltage representative of thedetected power level; means for producing a variable reference voltage;comparing means, operably connected to the detecting means and thevariable reference voltage, for comparing the supplied DC voltage to thevariable reference voltage; and means, operably connected to thecomparing means, for further controlling operation of the FET based atleast in part on a comparison of the supplied DC voltage and thevariable reference voltage.
 24. The RF amplifier circuit of claim 15further comprising: a first detecting means, operably connected to theRF input signal, for detecting the power level of the RF input signaland supplying a first DC voltage representative of the detected inputpower level; a second detecting means, operably connected to the RFoutput signal of the circuit, for detecting the power level of the RFoutput signal and supplying a second DC voltage representative of thedetected output power level; and an adjusting means, connected to thefirst detecting means, the second detecting means and the bias circuit,for automatically adjusting the drain current based at least in part ona comparison of the second supplied DC voltage and the first supplied DCvoltage by an amount necessary to maintain an essentially constant gain.25. The RF amplifier circuit of claim 15 further comprising: detectingmeans, operably connected to the RF output signal of the circuit, fordetecting the power level of the RF output signal and supplying a secondDC voltage representative of the detected output power level; and meansfor producing a variable reference voltage; adjusting means, connectedto the detecting means, the variable reference voltage means and thebias circuit, for automatically adjusting the drain current based atleast in part on a comparison of the supplied DC voltage and thereference voltage to maintain an essentially constant output RF power.